The present invention relates to pumps having suction and discharge valves such as fluid production pumps of the type often referred to as a bottom hole pump. In more detail, the present invention relates to a centered-anchored, rod actuated downhole pump which is particularly useful for production from shallow oil wells of the type known as stripper wells because of its immunity to the sanding, gas lock, and other problems which typically characterize the bottom hole pumps which are commonly used for oil and gas production.
Although reference will be made throughout this specification of the use of a pump constructed in accordance with the present invention in an oil well, and particularly in a stripper well, it is not intended that the application of the present invention be so restricted. Many surface pumps having suction and discharge valves used for, for instance, fluid production or the pumping of mud or cement, are prone to the same problems of gas lock and/or the sticking of the valve(s) in an open position as a result of the lodging of particulate matter in that valve as are downhole pumps. The following description of a center-anchored, rod actuated, submerged pump constructed in accordance with the present invention is, therefore, considered an exemplary application of the apparatus of the present invention described for the purposes of complying with the disclosure requirements of the Patent Statute, it being understood that the scope of the invention is not so restricted.
Gas lock occurs in virtually all wells, but is especially common in stripper wells, e.g., those wells which are approximately 1000 feet or less in depth. In such wells, the fluid weight in the production tubing may be, for instance, about 400 p.s.i. against the traveling valve of the downhole pump as the piston is on the upstroke lifting fluid to the surface. That 400 p.s.i. remains against the traveling valve as the piston reverses directions. During the upstroke, fluid (oil and gas) enters the barrel of the pump as a result of the relief of pressure against the standing valve such that the standing valve opens to allow fluid to enter the barrel. The fluid in the barrel is compressed by that 400 p.s.i. on the downstroke until the pressure in the barrel causes the traveling valve to open to allow fluid to enter the production tubing and stay open until the piston reaches the bottom of the stroke and reverses. Upon reversal, the traveling valve closes, trapping fluid in the production tubing.
As long as there is sufficient fluid in the barrel of the pump, the commercially available pumps known to Applicant work very well, but when the well pumps off and only a small amount of fluid enters the barrel through the standing valve during the upstroke, or when a small amount of fluid and a large quantity of gas enter the barrel, the pressure that accumulates on the downstroke, for instance, 380 p.s.i., does not exceed the 400 p.s.i. needed to open the traveling valve. When the piston reverses, the 380 p.s.i. trapped between valves expands, keeping the standing valve closed, until the pressure in the barrel is lower than the pressure in the well. If the well is pumped off, there is no fluid in the casing and the standing valve stays closed, no fluid enters the barrel during the up-stroke, and the 380 p.s.i. is simply "re-compressed" on the downstroke. A pump in this condition is said to be "gas locked". The pump remains gas locked until either the fluid pressure in the casing rises to a level high enough to overcome the pressure in the barrel or something is done on the surface to unlock the pump.
Other problems are common to such pumps. For instance, both standing and traveling valves often stick in the open position. The sticking of the valves is a result of their ball and cage construction, which makes them susceptible to the lodging of particulate matter between the ball and the valve seat. It is not uncommon for the pump itself to stick and/or the barrel as a result of sand and other particulate matter becoming caught between the barrel and the plunger, the tolerances of which are close so as to effect a seal between plunger and barrel, and if sand lodges therebetween, either the plunger or barrel will be cut or the plunger sticks in the barrel. The structure of such pumps makes them particularly prone to such damage because such pumps rely on a seal which is formed between plunger and barrel by the leading edge of the plunger. Of course it is on the downstroke when the most pressure is exerted on that seal, and the location of that seal on the leading edge of the plunger causes the fluid, and the particulate matter suspended therein, to tend to be forced into the space between barrel and plunger as a result of that pressure. Further, the requirement of precise tolerances between plunger and barrel increases the cost of manufacturing such pumps and makes them difficult to refurbish and maintain.
Another common problem, referred to as "fluid pound", is a distinct, non-metallic jarring felt in the pull rod part way down the stroke. This problem results from partial filling of the barrel of the pump during the upstroke of the plunger. When partially filled, the fluid in the tubing will follow the traveling valve down and, when the traveling portion of the pump does contact the fluid, it momentarily all but stops its motion, and the momentum of the entire column of fluid in the tubing aids in keeping the traveling valve momentarily closed. Stopping this fluid suddenly develops severe hydraulic shock, similar in character to the "water hammer" that occurs if a plug valve suddenly cuts off the flow of water in a long line. The effect of this shock is transmitted through the traveling assembly of the pump, causing a severe shock wave in the portion of the pump between the standing and traveling valves. This shock wave can attain forces several times that of the static pressure in the tubing column, and when it occurs near the middle of the stroke, the plunger is reaching its maximum velocity and the magnitude of the pound is most severe. Naturally, the pressure increase of this shock wave opens the traveling valve and the force of the shock is immediately dissipated in the larger volume of fluid in the tubing.
Fluid pound is naturally more severe in deep wells because of the higher pressure and longer column of fluid that is in motion, or in larger bore pumps where the mass of fluid in motion is larger, but affects pumps usable in wells of any depth. Although pumps are surprisingly rugged, the cumulative fatigue effects of fluid pound in the pump barrel, the rod string, and the pumping unit cannot be ignored. The barrel of a top anchored pump has the poorest resistance to fluid pound since the shock pressure generated in the lower portion of the barrel has only the relatively low pressure of the fluid in the well bore at suction pressure acting on the outside of the barrel. Severe fluid pound should, therefore, be specifically avoided in top anchored pumps.
Even this short description of some of the problems which are common to conventional downhole pumps highlights the difficulties encountered when the pump is used in stripper wells. Such wells are often sporadic or slow producers of oil, and are therefore prone to being pumped off, and often produce varying quantities of oil and/or gas such that gas lock is a particularly common problem. The fluids produced by such wells often include large quantities of sand and other particulates which can foul the pump. Further, even though they are generally shallow, various pressure conditions and depths are encountered in different stripper wells such that the choice of pump for a particular stripper well often is a choice between pumps having the fewest disadvantages. There is, therefore, a need for a downhole pump which overcomes these tendencies for use in such wells and it is a principal object of the present invention to provide such a pump.
The choice of rod actuated pump for use in a stripper well is generally a choice made between three types of pump: